This application claims the benefit of Korean Patent Application No. 2000-1403, filed on Jan. 12, 2000, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a multi-domain liquid crystal display device in which a common auxiliary electrode is formed around and in a pixel region on the same layer as a gate line, and an electric field induction window and a dielectric structure are formed in the pixel region.
2. Discussion of the Related Art
Among flat-panel displays enjoying image quality equivalent of the one offered by cathode ray tubes (CRT), liquid crystal displays (LCD) have been most widely adopted. In particular, a thin-film transistor (TFT) type LCD (TFT-LCD) has been used in personal computers, word processors, office automation equipment, and home electrical appliances including, among other items, a portable television sets. The market for suck LCDs is expected to further expand in the future. Accordingly, there is a demand for further improvement in image quality.
A description will be made by taking the TFT LCD for instance. However, the present invention is not limited to the TFT LCD but can apply to a simple matrix LCD, a plasma addressing type LCD and so forth. Generally, the present invention is applicable to LCDs which include liquid crystal sandwiched between a pair of substrates on which electrodes are respectively formed and displays images by applying voltage between the electrodes.
Currently, a mode most widely adopted for the TFT LCD is a normally-white mode that is implemented in a twisted nematic (TN) LCD. The technology of manufacturing the TN TFT LCD has advanced considerably in recent years. Contrast and color reproducibility provided by the TN TFT LCD have surpassed those offered by the CRT. However, the TN LCD has a critical drawback of a narrow viewing angle range, which limits the application of the TN LCD.
FIGS. 1A to 1C are diagrams for explaining the narrow viewing angle problem. FIG. 1A shows a state of white display to which no voltage is applied and liquid crystal molecules are aligned in the same direction with a slight inclination (about 1xc2x0 to 5xc2x0). For convenience sake, the liquid crystal molecules are illustrated as in FIG. 1A. In this state, light is seen nearly white in any azimuth. Moreover, as shown in FIG. 1C when voltage is applied, intermediate liquid crystal molecules, except those located near the alignment films, are aligned in a vertical direction. Incident linearly-polarized light is therefore seen black, but not twisted. At this time, the direction of light obliquely incident to a screen (panel) is twisted to some extent because it passes obliquely through the liquid crystal molecules that are aligned in the vertical direction. The light is therefore seen halftone (gray), but not perfect black.
As shown in FIG. 1B, when an intermediate voltage, which is lower than the voltage applied in the state shown in FIG. 1C, is applied, the liquid crystal molecules near the alignment films are aligned in a horizontal direction, but the liquid crystal molecules in the middle parts of cells erect themselves halfway. The birefringent property of the liquid crystal is lost to some extent. This causes transmittance to deteriorate and brings about halftone (gray) display. However, this effect occurs only to light incident perpendicularly on the liquid-crystal panel. Obliquely incident light is seen differently, that is, light is seen differently depending on whether it is seen from the left or right side of the drawing (panel). As illustrated, the liquid crystal molecules are aligned mutually parallel relative to light propagating from right below to left above. The liquid crystal hardly exerts a birefringent effect. Therefore, when the panel is seen from left, it is seen black. By contrast, the liquid crystal molecules are aligned vertically relative to light propagating from below on the right to above on the left. The liquid crystal exerts a great birefringent effect relative to incident light, and the incident light is twisted. This results in nearly white display. Thus, the most critical drawback of the TN LCD is that the display state varies depending on the viewing angle.
It is known that viewing angle performance of a liquid crystal display device (LCD) in the TN mode can be improved by setting the orientation directions of the liquid crystal molecules inside pixels to a plurality of mutually different directions. Generally, the orientation direction of the liquid crystal molecules (pre-tilt angles) that keep contact with a substrate surface in the TN mode are restricted by the direction of a rubbing treatment applied to the alignment film. The rubbing treatment is a process which rubs the surface of the alignment film in one direction by a cloth such as rayon, and the liquid crystal molecules contacting the alignment film are orientated in the rubbing direction. Therefore, viewing angle performance can be improved by making the rubbing direction different inside the pixels.
FIGS. 2A to 2C show a method of making the rubbing direction different inside the pixels. As shown in this drawing, an alignment film 101 is formed on a glass substrate 100 (whose electrodes, etc., are omitted from the drawing). This alignment film 101 is then bought into contact with a rotating rubbing roll 102 to execute the rubbing treatment in one direction. Next, a photo-resist is applied to the alignment film 101 and a predetermined pattern is exposed and developed by photolithography. As a result, a patterned layer 103 of the photo-resist is formed as shown in FIGS. 2B and 2C. Next, the alignment film 101 is brought into contact with a rubbing roll 201 that is rotating in a direction opposite the direction of the previous rotating rubbing roll 102, so that only the open portions of the pattern are rubbed. In this way, a plurality of regions that are subjected to the rubbing treatment in different directions are formed inside the pixel, and the plural orientation directions of the liquid crystal are formed inside the pixel. Incidentally, the rubbing treatment can be done in arbitrarily different directions when the alignment film 101 is rotated relative to the rubbing roll 102.
As described above, there are some problems related to a division of orientation directions of the liquid crystal molecules for improving the viewing angle performance in the VA LCD.
It is desirable to improve a viewing angle characteristic of a VA liquid crystal display, and to realize a VA liquid crystal display exhibiting a viewing angle characteristic that is as good as or better than that exhibited by an in-plane switching (IPS) mode LCD, while permitting the same contrast and operation speed as the conventional liquid crystal displays.
According the an embodiment of the present invention, a VA mode LCD uses a conventional vertical alignment film, and adopting a negative liquid crystal, and a domain regulating means for regulating the orientation of a liquid crystal. Liquid crystal molecules are aligned obliquely when a voltage is applied, so that the orientation will include a plurality of directions within each pixel. The domain regulating means is provided on at least one of the substrates. Further, at least one of domain regulating means has inclined surfaces (slopes). The inclined surfaces include surfaces which are almost vertical to the substrates. Rubbing need not be performed on the vertical alignment film.
In the VA LCD device, when no voltage is applied, in almost all regions of the liquid crystal other than the protrusions, liquid crystal molecules are aligned nearly vertically to the surfaces of the substrates. The liquid crystal molecules near the inclined surfaces also orientate vertically to the inclined surfaces. When a voltage is applied, the liquid crystal molecules tilt according to electric field strength. Since the electric fields are vertical to the substrates, when a direction of tilt is not defined by carrying out rubbing, the azimuth in which the liquid crystal molecules tilt includes all directions of 360xc2x0. If there are pre-tilted liquid crystal molecules, surrounding liquid crystal molecules are tilted in the directions of the pre-tilted liquid crystal molecules. Even when rubbing is not carried out, the directions in which the liquid crystal molecules lying in gaps between the protrusions can be restricted to the azimuths of the liquid crystal molecules in contact with the surfaces of the protrusions. When voltage is increased, the negative liquid crystal molecules are tilted in directions vertical to the electric fields.
Recently, a liquid crystal display device which drives a liquid crystal by an auxiliary electrode, which is electrically insulated from a pixel electrode, without aligning the liquid crystal has been suggested. Such a related art liquid crystal display device will be described with reference to FIG. 3.
As shown in FIG. 3, the related art liquid crystal display device includes a first substrate, a second substrate, a plurality of data lines and gate lines, a thin film transistor, a passivation film 37, a pixel electrode 13, and an auxiliary electrode 21. The data lines and gate lines are formed on the first substrate lengthwise and crosswise to divide the first substrate into a plurality of pixel regions. The thin film transistor is formed in each pixel region on the first substrate and includes a gate electrode, a gate insulating film, a semiconductor layer, an ohmic contact layer, and source/drain electrodes. The passivation film 37 is formed on the first substrate. The pixel electrode 13 is formed on the passivation film 37 to be connected with the drain electrode. The auxiliary electrode 21 is formed on the gate insulating film to partially overlap the pixel electrode 13.
The related art liquid crystal display device further includes a light-shielding layer 25, a color filter layer 23 formed on the light-shielding layer 25, a common electrode formed on the color filter layer 23, and a liquid crystal layer formed between the first substrate and the second substrate. The light-shielding layer 25 is formed on the second substrate to shield light leaked from the gate lines, the data lines, and the thin film transistor.
The auxiliary electrode 21 formed around the pixel electrode 13 and an open region 27 of a common electrode 17 distort electric field applied to the liquid crystal layer so that liquid crystal molecules are variously driven within a unit pixel. This is intended that a dielectric energy by the distorted electric field places a liquid crystal director at a desired position when a voltage is applied to the liquid crystal display device.
However, the liquid crystal display device requires the open region 27 in the common electrode 17 to obtain multi-domain effect. To this end, the process for patterning the common electrode is additionally required.
Furthermore, if the open region 27 is not formed or has a small width, distortion range of the electric field required to divide the domain is weak. Accordingly, there is a problem that, the time required for the liquid crystal director to reache a stable state, relatively increases. Such a domain division by the open region 27 causes unstable textures for each domain, thereby deteriorating picture quality. Also, because high electric field is applied between the pixel electrode 13 and the auxiliary electrode 21, luminance and response speed decrease.
Accordingly, the present invention is directed to a multi-domain liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide provide a multi-domain liquid crystal display device in which a common auxiliary electrode is formed around and in a pixel region on a layer equal to a gate line, and electric field induction windows and dielectric structures are formed in the pixel region, so that stable texture and multi-domain effect can be obtained.
A multi-domain liquid crystal display device of the present invention is an improved invention of the Korean Patent Application No. 1999-07633 filed by the same applicant of this invention, in which a common auxiliary electrode is formed around a pixel region on a layer equal to a gate line, and electric field induction windows and dielectric structures are formed in the pixel region.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a multi-domain liquid crystal display device according to the present invention includes: first and second substrates opposing each other; a liquid crystal layer formed between the first substrate and the second substrate; a plurality of gate lines and data lines formed on the first substrate lengthwise and crosswise to define pixel regions; a pixel electrode formed in the pixel regions; at least one or more electric field induction windows independently formed in the pixel electrode; a common auxiliary electrode formed on a layer equal to the gate lines to surround the pixel regions; a common electrode formed on the second substrate; at least one or more dielectric structures independently formed on the common electrode to distort electric field applied to the liquid crystal layer; and an alignment film formed on at least one of the first and second substrates. The multi-domain liquid crystal display device further includes a common auxiliary electrode formed in a region where the electric field induction windows are formed, and the dielectric structures are formed to maintain a cell gap of the liquid crystal display device.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.